The conservative Tea Party Express praised Mitt Romney’s decision to tap Ryan “who is so popular with tea party activists.”

And Sen. Marco Rubio, the Florida Republican who also had been a vice presidential contender, called Ryan a “courageous reformer.” Former presidential candidate Rick Santorum said adding Ryan to the ticket reflects Romney’s commitment to restoring “fiscal sanity,” and former Florida Gov. Jeb Bush found the choice “courageous.”

The conservative Club for Growth said the pick “sends the message that Gov. Romney is interested in bold reforms.”

Democrats and their allies, though, welcomed the opportunity to bring the Ryan fiscal agenda to voters this fall, and immediately sent out a fundraising email about Romney’s choice of “the architect of the Republican plan to kill Medicare.”

“Congressman Paul Ryan and Mitt Romney are a match made in millionaires’ heaven,” said Rep. Steve Israel of New York, who directs Democratic campaign efforts in the House. “But they’ll be a nightmare for seniors who’ve earned their Medicare benefits.”

Ryan, a Wisconsin congressman who chairs the House Budget Committee, has long been a polarizing figure in national politics for his austere approach to reining in the nation’s deficits and revamping the entitlement system — particularly his Medicare proposal that would allow the next generation of seniors, those now 55 and under, the option of a voucher-like system to buy private insurance outside the existing senior health program. The 42-year-old is one of the so-called “Young Guns” of up-and-coming House Republican leaders.

“A Romney-Ryan ticket is a weapon of mass destruction unapologetically-pointed at our Social Security, our Medicare and our hopes for maintaining a modest, middle class lifestyle during our retirement years,” said Eric Kingson, co-founder of the Social Security Coalition, and Nancy Altman, co-director of the Social Security Works.

With the choice of Ryan, the presumed GOP presidential nominee has shown the American people that an “extremist, irresponsible and anti-worker agenda is what’s right for our country,” said Mary Kay Henry, the president of the SEIU labor union.

The Progressive Change Campaign Committee called Ryan “a right-wing extemist who wants to end Medicare.”

Rep. Kathy Hochul, the New York Democrat who won a 2011 special election in a race that experts said turned in part on Ryan’s proposed Medicare changes, campaigned on the choice Saturday, saying the country needs fresh choices not a rehashing of “failed ideas.”

Ryan also drew sharp responses from women’s groups over his position against abortion.

The anti-abortion Susan B. Anthony List praised his “pristine pro-life voting record,” and his insistence there be “no ‘truce’” on the issue.